what is your definition of "bad"? I assume your house is built as what's called "platform" framing. This means that the foundation has your joists sitting on it, then all of your walls (both inside and exterior load bearing walls) are sitting on the joists. Your second floor joists sit on top of the first floor walls, and so on, up to the roof. In most cases, it isn't simply a matter of yanking a joist out and replacing it. The reason for doing it is also important. You don't say if your joists are bad because they warped, you cut them in half, you drilled a bunch of holes in them where you weren't supposed to, or you never put gutters on the house so the water that ended up in your crawlspace rotted the joist to the point where your first floor is about to drop 4 feet.
Okay, guys, quit laughing.
Your easiest solutions, assuming the joists are somewhat salvageable is to sister a good joist next to the existing ones if the joist is structuraly unsound. If it is just a matter of a joist that pulled away from the plane of the floor and now you have a soft spot with a floor squeak, a few shims are all you need.
If the joists are all rotten, you have a serious problem that you want to explore with your home insurance agent, several qualified contractors, and a structural engineer.
Post back with some more specific information, (where you live, what the definition of bad is, how many joists are we talking about, what you think made the joists bad in the first place, etc.) and someone should be able to help you more specifically. Be PAINFULLY descriptive of the situation.